This invention relates to methods and circuits for processing and filtering signals. Such methods and circuits can be particularly adapted to reduce amplitude variations in angle modulated radio frequency signals, or to provide comb notch filtering that removes interference of a repetitive nature.
Multipath phenomena results from multiple paths taken by a transmitted radio frequency signal before the signal reaches a receiver. Transmission of the signal along these paths results in constructive and destructive interference which causes amplitude variations that are a function of the signal's instantaneous frequency. These variations in amplitude complicate subsequent signal processing. For example, in the reception of radio frequency signals, a situation is frequently encountered in which a weak signal of interest (SOI) is subject to interference by a strong interfering signal having a frequency band which encompasses that of the SOI. The interfering signal may be, for example, a jamming signal or a commercial radio or television signal. If the interfering signal is an angle modulated signal, variations in amplitude whether caused by multipath phenomena or otherwise, can further mask the signal of interest, making detection of the signal of interest more difficult.
Interference of a repetitive nature can also cause harmonic interference, in which case it is desirable to remove the harmonic components of a signal. A comb notch filter may be used for such purposes.
The present invention seeks to provide a method and circuit for processing radio frequency signals to filter the signals or to remove the effects of amplitude variations wherein the amplitude variations are correlated with the instantaneous frequency of the signal.